The day of Antietam—September 17, 1862 — was like no other day of the Civil War. “The roar of the infantry was beyond anything conceivable to the uninitiated,” wrote a Union officer who fought there. “If all the stone and brick houses of Broadway should tumble at once the roar and rattle could hardly be greater … and amidst this, hundreds of pieces of artillery, right and left, were thundering as a sort of bass to the infernal music.” Over the course of 14 hours of this unceasing roar and rattle, 22,700 Northerners and Southerners were killed, wounded, or listed as missing—the worst one-day toll of the war, indeed the worst loss of life in a single day in America’s history.

Yet at the end of that indescribably bloody September 17, neither side had won and neither had lost. A Northern war correspondent called it, optimistically, “partly a success; not a victory, but an advantage had been gained.” A Southern correspondent said the Confederates grudgingly accepted it as a drawn battle only “because they had not in their usual style got the enemy to running.” Militarily it was a day of wrenching missed opportunities. But in fact no battle of the Civil War—not Gettysburg, not Vicksburg, not Missionary Ridge—was in the end more meaningful than Antietam. Neither of the two army commanders—Robert E. Lee and George B. McClellan—had planned to fight this battle where and when they did. Two weeks earlier, Lee had led his Army of Northern Virginia across the Potomac into western Maryland to fight a showdown battle on ground of his choosing in Maryland or Pennsylvania. General Lee, according to the notes of a postwar interviewer, said he intended to “have had all my troops reconcentrated on the Md. side, stragglers up, men rested & I intended then to attack McClellan, hoping the best results from state of my troops & those of enemy.” General McClellan was more inclusive and less definite: “Again I have been called upon to save the country,” he wrote his wife on September 5; “the case is desperate, but with God’s help I will try unselfishly to do my best & if he wills it accomplish the salvation of the nation.”

Lee was on a winning streak. In the late spring, in the titanic Seven Days Battles on the Virginia Peninsula, he drove McClellan and his Army of the Potomac away from the gates of Richmond. By September Lee was at the gates of Washington, having roundly defeated a second Union army under John Pope in the Second Battle of Bull Run. Lee cast his eyes north. He regarded a short war as the Confederacy’s best route to victory, and he wrote Jefferson Davis that now was the time to propose to the United States “the recognition of our independence … when it is in our power to inflict injury upon our adversary.” He intended to deliver that injury on Northern soil against the North’s principal army. He hoped that his army, simply by the act of invading Northern soil, would influence the approaching midterm elections and increase support for a Northern peace party.

That first week of September found the Lincoln administration caught up in a command crisis. In August McClellan had been ordered up from the peninsula to combine his army with Pope’s to counter Lee’s advance. But the Young Napoleon (as the press dubbed McClellan) clung to his Peninsula Campaign. He despised John Pope and willfully dragged his feet, holding back two army corps that could have made the difference in Pope’s fight at Bull Run. This conduct infuriated the president. “Unquestionably he has acted badly toward Pope!” he told his secretary. “He wanted him to fail. That is unpardonable.” But he added, “He is too useful now to sacrifice.” Against his cabinet, which petitioned him to sack McClellan, against even his own best instincts, Lincoln replaced Pope with McClellan and gave him command of the combined armies. The deciding factor was the demoralized state of the troops. It was feared that just now the army would not fight for anyone but the Young Napoleon.

The modern term “charismatic” nicely describes George McClellan. His men admired him, and he cultivated their admiration with grand reviews and ringing proclamations. He was a scholar of the Napoleonic Wars (studied in the original French) and struck Napoleonic poses. Yet on the peninsula he was not to be seen on the battlefields. “Curiously enough,” wrote soldier-historian Francis W. Palfrey, “there was almost always something for McClellan to do more important than to fight his own battles.” The pending battle in Maryland would be a first for the Young Napoleon.

McClellan’s particular skill was military organization, now apparent in his gathering up of the disparate commands from the disastrous Second Bull Run campaign, reorganizing and refitting them, and restoring morale. In a remarkably short time the reconstituted Army of the Potomac was ready to fight again. Reports placed the Confederate army at Frederick in Maryland, and McClellan set his own army in motion to follow. He moved with great circumspection. By his accounting he was (once again) heavily outnumbered—the Confederates, he claimed, were 120,000 strong, surpassing his army by at least 30,000 men. McClellan had assumed this underdog role from the moment he took command of the Army of the Potomac in August 1861, and that stance had shaped his every strategic and tactical decision since. But it was and had always been a grand delusion. It was his army that was invariably the stronger, by a factor of two or even three. Such was the case in Maryland. At Antietam McClellan had roughly two and a half times Lee’s manpower. He would wage war against a phantom Confederate army.

It was Lee’s plan to march west from Frederick and force a battle in the broad Cumberland Valley of Maryland and Pennsylvania, pulling the Federals away from their Washington base while he drew his own supplies through the Shenandoah Valley. But despite being cut off by the Confederates’ advance, the Federals did not evacuate Harpers Ferry, guarding the mouth of the valley where the Shenandoah joined the Potomac. Lee elected to pause his campaign and sweep down on Harpers Ferry from three directions. Concealing his movements behind the bulk of South Mountain, he expected to complete the capture before McClellan realized what was going on. He issued his orders on September 9, and the next day the Confederate army left Frederick and marched off in the designated directions.

But on September 13 Lee’s best-laid plan was dangerously compromised when a Federal soldier came upon a copy of the Harpers Ferry plan, lost by a careless courier. The so-called Lost Order was soon in McClellan’s hands. In an exuberant telegram to Lincoln he announced, “I have all the plans of the Rebels and will catch them in their own trap … . Will send you trophies.”

The Lost Order was the intelligence coup of the war. The Confederate army was divided into five well-scattered columns, and once McClellan broke through the South Mountain passes, he would be perfectly situated to divide and conquer.

On September 14 the Confederates had just time enough to cobble together a delaying defense at Turner’s Gap and Crampton’s Gap in South Mountain. Still, Lee had decided to give up his campaign until word reached him from Stonewall Jackson that “Through God’s blessing” Harpers Ferry was in his grasp, “and I look to Him for complete success tomorrow.” Lee changed his mind and set an assembly point for his scattered army at the sleepy Maryland village of Sharpsburg, between Antietam Creek and the Potomac River.

Fully expecting the Confederates to flee back to Virginia after the beating they took at South Mountain, McClellan was surprised by a dispatch from a signal officer the next day: “A line of battle—or an arrangement of troops which looks very much like it—is formed on the other side of the Antietam creek and this side of Sharpsburg. It is four times longer on the west than on the east side of the road.”

McClellan found it too late that day to attack, and when he learned that Harpers Ferry had fallen to Jackson, he concluded that he must now be facing the entire Confederate army. “I have the mass of their troops to contend with,” he explained to Washington, “& they outnumber me when united.” So he spent September 16 arranging his forces and plotting a plan of attack. As firmly in the grip of delusion as ever, he never imagined that Lee was bluffing him on September 15–16 with hardly 15,000 men of all arms. The first of Jackson’s troops only began to reach Sharpsburg from Harpers Ferry on the afternoon of the 16th; at day’s end Lee was still short three of his nine divisions. Even after those three arrived on the day of battle, he had barely 40,000 troops on the field. McClellan had available to him in the Army of the Potomac 101,000 men.

The capture of Harpers Ferry netted the Confederates 12,700 prisoners, 73 pieces of artillery, 13,000 small arms, and abundant supplies—surely triumph enough for Lee to fall back to Virginia without disgrace and recalculate his plans. Instead he elected to risk a stand at Sharpsburg. He had whipped the Young Napoleon before Richmond, chased him off the peninsula, and beaten a good part of his army (along with Pope’s), at Second Bull Run. Lee was not going to be run out of Maryland before a test of arms against a general he characterized as timid.

McClellan had wasted all the opportunities inherent in the Lost Order except for this: Lee would be fighting this battle before he wanted to and not where he had first intended. For his part, McClellan would have to do battle on ground not of his choosing, and he would have welcomed more time to reorganize and refit his patchwork army. But Lee had thrown down the gauntlet in challenge, and McClellan elected Wednesday, September 17, as the day of battle.

The Confederate line roughly followed a low ridge that ran north-south down the middle of the irregular peninsula formed by Antietam Creek and the Potomac. Sharpsburg lay behind the ridge, its church steeples visible to the Federals east of the creek. McClellan aimed his primary attack against the left or northern end of the Confederate line, where the landscape of crop fields and meadows and woodlots appeared best suited for fighting. Late on Tuesday “Fighting Joe” Hooker led his First Corps across the Antietam and took up an attacking position, supported by General Joseph K. F. Mansfield’s Twelfth Corps. Farther to the south, where the landscape became steeper and more broken, Ambrose Burnside’s Ninth Corps was posted for a secondary attack. McClellan retained half his army—the Second, Fifth, and Sixth Corps—at his center east of the creek.

At first light on September 17, Hooker pushed south through a 30-acre cornfield, the East Woods on his left and the West Woods on his right. His target was the small whitewashed church of the German Baptist Brethren, called Dunkers for their baptism by total immersion, that sat on an open plateau crowded with Confederate artillery. Stonewall Jackson had turned his flank into a solid front. When the Yankees burst out of the corn, wrote a Wisconsin soldier, “a long line of men in butternut and gray rose up from the ground. Simultaneously, the hostile battle lines opened a tremendous fire upon each other. Men, I cannot say fell; they were knocked out of the ranks by dozens.” Quickly the fighting spread from the East Woods through the cornfield to the West Woods, with terrible casualties.

Lee sent in John Bell Hood’s division in a counterattack that drove back Hooker’s corps. Hooker countered with Mansfield’s Twelfth Corps, and although General Mansfield fell mortally wounded, his corps, now under Alpheus Williams, gained a foothold at the Dunker church and achieved a momentary stalemate on this northern battlefront. McClellan had assured Hooker that the rest of the army would attack in concert with him, yet he did not call up General Edwin Sumner’s powerful Second Corps to reinforce the attack for nearly two hours. When the order finally came, Sumner had two miles to march to reach the fighting.

General Sumner was 65, a gallant old warrior but, as McClellan put it, “nature had limited his capacity to a very narrow extent.” McClellan intended him to arrive as support for the attack and thus be under Hooker’s direction, but Hooker fell wounded, and abruptly Sumner was in command of the northern front. McClellan held back the third of Sumner’s three divisions to bolster his defenses against the counterstroke he expected from the Confederate host, and the second of Sumner’s divisions lagged behind and wandered off course. With only a single division, led by General John Sedgwick, but determined to turn the enemy’s flank, Sumner marched in three lines of battle straight into the West Woods.

General Williams of the Twelfth Corps warned that the enemy was in force in the West Woods, but Sumner brushed off the warning, and Williams watched horrified as Sedgwick’s division “made, without halt or reconnaissance, straight for the woods.” Lee, in a perfect reading of the battlefield, had ordered one of his divisions just up from Harpers Ferry to reinforce Jackson, and with explosive force it slammed into the left flank of the Federal advance.

Sedgwick’s three brigade lines were hardly 50 yards apart and unable to pivot to meet the assault. In moments 600 men fell, and the lines crumpled like rows of dominoes. A Massachusetts soldier reported that “the left Regiments gave way in confusion, the enemy poured in upon our rear, and now the slaughter was worse than anything I have ever seen before.” The rout in the West Woods ultimately cost the Federals over 2,300 casualties and all but ended the fighting on the northern battlefront.

The Second Corps division that had gone astray stumbled into a new battlefront at Lee’s center. The Confederate line here followed a farm lane that erosion and travel had worn down to form a natural trench, to be known as the Sunken Road. The Federal commander, unimaginative William H. French, could think of nothing better than serial frontal attacks against the Sunken Road position. A North Carolina colonel said his volleys “brought down the enemy as grain falls before a reaper.” McClellan finally released Israel Richardson’s Second Corps division, which joined French in the assault. Lee threw in his last reserves to brace his endangered center.

Richardson’s forces overlapped the enemy line, and suddenly the Sunken Road became a death trap for its defenders. Lee’s center fell into collapse, held only by a line of guns. Richardson called for reinforcements, especially in artillery, but his call went unanswered. Then he was mortally wounded, and his fall marked the end of the fight for the center. McClellan, blind to opportunity, was content to stand on the defensive. Lee was reprieved.

Lee was soon reprieved a second time when McClellan rode to the right to consult with his generals. The Sixth Corps had now reached the field, and its commander, William Franklin, proposed leading a renewed battle there. But old Sumner was demoralized, insisting that any further fighting on this front would risk “a total defeat.” McClellan would not overrule him, saying he “was afraid to risk the day by an attack there on the right at this time.”

“Afraid to risk”—the epigraph for George McClellan’s generalship.

One act remained before darkness ended the fighting. Ambrose Burnside, at the southern end of the field, was the only Federal general that day to face a contested Antietam crossing. On the west the ground fell steeply to the creek bank, and the Confederate defenders had the narrow stone bridge there under point-blank fire. It took three hours and three costly attacks before the bridge was taken. But by afternoon the Ninth Corps was advancing steadily on Sharpsburg and threatening Lee’s sole line of retreat. Then came a devastating surprise—A. P. Hill’s division, the final piece of Lee’s army, arriving after a hard march from Harpers Ferry, crashed into Burnside’s flank and drove his command back to the bridge. The surprise stemmed from one of McClellan’s more egregious errors of the day: He massed his cavalry behind his center, leaving the army’s flanks unguarded, so that Hill’s attack seemingly came out of nowhere.

September 18 found Lee standing defiantly in his lines and McClellan making no move to resume the battle, and that night the Army of Northern Virginia crossed the Potomac back into Virginia. The Federals’ modest attempt at pursuit was hurled back by Lee’s rear guard. McClellan announced that Maryland and Pennsylvania were safe from the invaders and credited himself with a great victory against long odds. But in truth he had not fought to win, only to avoid losing (he engaged only 11 of his 17 divisions), and consequently Robert E. Lee escaped a potentially crushing defeat. Soldier-historian Ezra Carman wrote of Antietam: “History will not accept [McClellan’s] view of the battle, in the conduct of which more errors were committed by the Union commander than in any other battle of the war.”

McClellan’s casualties on September 17 exceeded Lee’s, 12,400 to 10,300, and over the course of the Maryland campaign he lost 27,000 (including the Harpers Ferry captives) against Lee’s 14,000. In the weeks following Antietam the Young Napoleon’s continued posturing and maddening delays finally led the president to dismiss him.

The battle had other consequences. The fact that the Confederates were back in Virginia rather than in Maryland or Pennsylvania helped the Lincoln administration in the fall elections; despite electoral losses, the Republicans kept control of Congress. Of greatest importance, on September 22 Lincoln called his cabinet together to announce he was issuing the preliminary Emancipation Proclamation. He had been awaiting a battlefield victory to make it public; otherwise it might seem an act of desperation in a losing cause.

“The action of the army against the rebels has not been quite what I should have best liked,” the President told the cabinet, but the danger of invasion was over. He said he “had made a vow, a covenant, that if God gave us the victory in the approaching battle, he would consider it an indication of Divine will” and would move forward on emancipation. “God had decided this question in favor of the slaves.” On January 1, 1863, the slaves in any state in rebellion, he declared, “shall be then, and thenceforward, and forever free.”

Antietam and the resultant emancipation also ensured that the South would fight the war without friendly foreign intervention. The British and French had been leaning toward a role in the war—mediation, recognition of the Confederacy, perhaps military assistance—and setting considerable store by Lee’s string of victories. When it became a war to free the slaves as well as to restore the Union, no country would dare enter on the side of a slavery regime.